The application relates generally to refrigeration, air conditioning and chilled liquid systems. The application relates more specifically to methods of operating refrigeration, air conditioning and chilled liquid systems.
It has been recognized that under certain environmental conditions and reduced system cooling demand conditions, chilled liquid systems using a centrifugal compressor can be operated at a fraction of the cost when compared to the cost during normal operation, sometimes referred to as “free cooling”. The 2008 ASHRAE Handbook—HVAC Systems and Equipment (page 42.12) provides as follows:
Cooling without operating the compressor of a centrifugal liquid chiller is called free cooling. When a supply of condenser water is available at a temperature below the needed chilled-water temperature, some chillers can operate as a thermal siphon. Low-temperature condenser water condenses refrigerant, which is either drained by gravity or pumped into the evaporator. Higher-temperature chilled water causes the refrigerant to evaporate, and vapor flows back to the condenser because of the pressure difference between the evaporator and the condenser.
In other words, when the entering condenser water temperature is less than the exiting water temperature from the evaporator of a liquid centrifugal liquid chiller, and when the demand for cooling is sufficiently low such that the exiting evaporator water temperature satisfies the demand for cooling, the centrifugal compressor is shut off, resulting in substantially energy savings.
Unfortunately, such environmental conditions in numerous parts of the world relatively rarely occur, or may be brief in duration. Occurring yet more rarely is the combination of the advantageous environmental conditions that simultaneously produce sufficient cooling output to satisfy the demand for cooling, in order to permit shut-down of the centrifugal compressor.
Thus, there is a need for a method of operating a chiller that significantly increases the range of environmental conditions (e.g., increases the range of temperatures of entering condenser water temperatures and exiting evaporator water temperatures as well as the range of differences therebetween) to achieve energy savings during chiller operation. There is a further need for a method of operating a chiller in the above-referenced range of environmental conditions that increases chiller load capacity while simultaneously achieving such energy savings.